Homebrew PCBs

I'm comfortable with the exposure and developing of photosensitive 
boards, but I'm curious how one goes about making them photosensitive in 
the first place?

I've seen positive and negative pre-treated boards for sale, but they 
tend to cost about 10x what I'm used to paying for PCB material.  Is 
treating them oneself worth the effort, or should I just go ahead and 
buy the ready-made boards?

Dave K